In the prior art of this type of gas laser, for example, a carbon dioxide gas laser apparatus, a pair of metal electrodes are arranged at the two sides of a glass discharge tube of a predetermined length and a DC high voltage of, for example, 10 kV to 30 kV, is applied between said pair of metal electrodes via a predetermined series resistor (for setting the operating point in the discharge characteristics), thus imparting a predetermined energy to the carbon dioxide gas (in actuality, a mixture of N.sub.2 -He-CO.sub.2 is used) existing in the discharge tube. When the transition of the energy level of the gas molecules excited to the predetermined energy level in this way to a predetermined lower energy level occurs, laser light of a predetermined wavelength is generated. The laser light successively generated in this way travels back and forth between the reflecting mirrors provided at the two ends of the discharge tube for a predetermined laser oscillation. Note that the above-mentioned gas medium existing in the discharge tube is continually circulated through a circulation pump and coolant heat exchanger. Further, part is exhausted and replaced with fresh gas.
However, in a gas laser producing apparatus excited by DC high voltage, the gap length becomes longer due to the metal electrodes being disposed in the longitudinal direction of the glass discharge tube at the two ends thereof, requiring a high voltage, as mentioned above, and therefore making ensurement of insulation of the power source apparatus itself difficult. Further, along with this, the power source apparatus becomes large in size. Further, under said high voltage, the controllability for controlling the discharge voltage and discharge current to predetermined values required for stabilization of the laser output becomes poor and a high resistance series resistor is required for setting the discharge characteristics to a predetermined operating point, as mentioned above, whereby the loss due to power consumption increases, and further, consumption of the metal electrodes provided in the discharge tube and fouling of the discharge tube due to sputtering thereof, and other problems arise.
On the other hand, as a means to resolve the above problems, there has been proposed a gas laser producing apparatus of the so-called silent discharge excitation method, using an AC power source with a frequency of less than several hundred kilohertz, instead of the above-mentioned DC power source. In this case, a pair of electrodes are disposed at an interval of about several tens of millimeters, for example, in a predetermined gas under reduced pressure. A dielectric is placed inside each of the said electrodes and an AC voltage in the above-mentioned frequency range is applied between the electrodes. That is, specifically speaking, a ceramic tube is used for the dielectric. A pair of metal electrodes are disposed opposing each other on the outer surface of the ceramic tube along the longitudinal direction thereof. An AC voltage in the above-mentioned frequency range is applied between the pair of metal electrodes. Note that in the same way as the above-mentioned DC power source drive, reflecting mirrors are provided at the two ends of the ceramic tube (laser tube) and the gas medium in the laser tube (in the same way as the above, use if made of N.sub.2 -He-CO.sub.2 gas) is circulated.
In this case, the ions or electrons ionized in the gas medium move to the electrode side of the predetermined polarity and accumulate on the insulator provided on the inside thereof. Each time the polarity of the AC voltage changes, they move to the opposing electrode side. Current flows intermittently only when such changes in polarity occur. Therefore, the power injected into the gas medium per unit volume increases, when the applied voltage is constant, since the higher the frequency of the AC voltage, the greater the current flowing between the electrodes (this point may be explained from the fact that the ceramic tube acts as a capacitive impedance with respect to the AC voltage and, therefore, the higher the frequency, the lower the impedance). Along with this, the output power also increases (conversely, the apparatus required for obtaining a set output power can be made smaller). However, the upper limit on the frequency at which the above-mentioned silent discharge occurs is several hundred kilohertz at the most.
Further, the ceramic tube used for gas laser producing apparatuses of the said silent discharge excitation type have a large dielectric loss angle (tan .delta.) and therefore the higher the frequency of the said AC power source, the greater the dielectric loss and thus the higher the temperature, which finally causes dielectric breakdown of the ceramic tube. Therefore, when a ceramic tube is used as the laser tube, due to the restriction on the withstand voltage, there is the problem that the power source frequency cannot be increased that much. Of course, as a countermeasure, it is conceivable to use a quartz tube or another dielectric with a low dielectric loss, but in this case, in said frequency range, the impedance of the dielectric becomes too high and sufficient injected power requested for laser generation cannot be obtained. Therefore, in the silent discharge region, it is essential to use the above-mentioned ceramic tube etc. as the dielectric.
Further, it has been proposed to use a so-called radio frequency of a frequency of, for example, 13.56 Megahertz or 27 Megahertz or even higher as the drive power source of the gas laser producing apparatus. That is, when using such a high frequency, the period of change of the polarity of voltage mentioned above becomes extremely short, so during that interval the ionized ions or electrons will not reach the predetermined electrode, but continually repeatedly travel back and forth between the electrodes. This switching of the charge movement causes a continuous high frequency current (continuous current advanced in phase from the high frequency voltage impressed) to flow between the electrodes, and discharge of a different mode from the above intermittant discharge is performed. In this case, the percentage to which the gas medium can be raised to the predetermined excitation level with respect to a predetermined injected power increases, and the efficiency of laser light generation can be raised. However, in the above-mentioned frequency region, it is difficult to make the large output power source required for this type of laser light producing apparatus (in particular the high frequency inverter portion) with, for example, transistors and other solid state elements. Use must be made of vacuum tubes. In the final analysis, despite the power source frequency being raised and the efficiency improved, there is the separate problem that the power source apparatus ends up large in size.
Further, when use is made of a drive power source of a radio frequency, use is also made of quartz tubes, with their low dielectric losses, as the laser tubes, but under such a high frequency, the impedance of the quartz tubes becomes too low and, therefore, unless the said quartz tube is made thick, the discharge current will end up concentrating at local areas and fabrication will become difficult. Further, the current passing through the tube wall of said thickness separate from the discharge space will increase, whereby the loss due to the same will increase, posing other problems. Further, in such frequency regions, problems occur of radio interference, and thus there are problems of higher costs due to the shielding etc. to deal with the same, the larger size of apparatuses, etc.